1. Field of the Invention
The present invention relates to a light emitting device using an organic compound, and more specifically, to an organic light emitting device from which light is emitted by applying an electric field on a thin film made of an organic compound.
2. Related Background Art
An organic light emitting device is a device that includes a thin film made of an emitting organic compound between an anode and a cathode, generates an exciton from the fluorescent organic compound by injection of an electron and a hole from each electrode, and utilizes light to be radiated when the exciton returns to the ground state.
A study conducted by Eastman Kodak Company in 1987 (Appl. Phys. Lett. 51, 913 (1987)) reported light emission in the order of 1,000 cd/m2 at an applied voltage of about 10 V, from a device having a separated-function type two-layer structure in which an anode is made of ITO and a cathode is made of a magnesium-silver alloy, an aluminum quinolinol complex is used as each of an electron-transporting material and a light emitting material, and also a triphenylamine derivative is used as a hole-transporting material. Related patents include U.S. Pat. Nos. 4,539,507, 4,720,432, and 4,885,211.
In addition, luminance at spectra ranging from ultraviolet through infrared is allowed by changing the type of the fluorescent organic compound. Recently, therefore, various compounds have been studied extensively and described in, for example, U.S. Pat. Nos. 5,151,629, 5,409,783 and 5,382,477, and Japanese Patent Application Laid-Open Nos. H2-247278, H3-255190, H5-202356, H9-202878 and H9-227576.
Furthermore, in addition to organic light emitting devices using low molecular weight materials as described above, an organic light emitting device using a conjugated polymer has been reported by a group from Cambridge University (Nature, 347, 539 (1990)). This report confirmed luminance from a monolayer by film formation with polyphenylene vinylene (PPV) in a coating system. Patents relating to an organic light emitting device using a conjugated polymer include U.S. Pat. Nos. 5,247,190, 5,514,878, 5,672,678, and Japanese Patent Application Laid-Open Nos. H4-145192 and H5-247460.
Recent advances in technology concerning organic light emitting devices have been remarkable. The characteristics of such devices allow the formation of thin and lightweight light-emitting devices having high luminance at a low applied voltage, a variety of emission wavelengths, and high-speed response, suggesting that these devices could be put to extensive use. However, many problems still remain to be solved in terms of durability, such as luminescence degradation over time by prolonged use.
In order to solve this problem, Japanese Patent Application Laid-Open No. 2002-43063 and U.S. Pat. No. 6,392,250 disclose a stacked-type organic light emitting device in which the emission layer is a mixed layer consisting of hole-transporting material, electron-transporting material and an emitter. This mixed layer suppresses the generation of unstable cationic species (holes) of the electron-transporting material, whose purpose is to improve operational durability. However, such a combination of a hole-transporting material and electron-transporting material has been often ineffective to improve the durability, and furthermore, no consideration has been given to deterioration of the emitter. In addition, these patent documents basically relates to an emission layer, so that a combination thereof with other layers has been often found ineffective.
Japanese Patent Application Laid-Open No. 2003-151777 discloses an organic light emitting organic device which employs a mixed layer of a hole-transporting material and an electron-transporting material, and is capable of further adding a light emitting material. In this device, selecting a material different from the mixed layer for the hole-transporting region or electron-transporting region provides the advantages of improving efficiency and stability due to blocking of charge leakage/exciton diffusion, increasing the degree of freedom in material selection, and increasing economic efficiency. However, the above advantages are merely the advantages of an ordinary stacked structure, and are not just limited to a device using a mixed layer as an emission layer. In addition, no consideration is given to deterioration of the emitting material.
Japanese Patent Application Laid-Open No. 2000-106277 discloses an organic light emitting organic device in which the emission layer contains a host material, a polycyclic aromatic hydrocarbon, and a fluorescent dye, wherein the hole mobility of the polycyclic aromatic hydrocarbon is greater than that of the host compound. This prior art is directed to prolonging the life of a device by suppressing hole accumulation in the host material of the emission layer by using a high hole-mobility polycyclic aromatic hydrocarbon. However, because the highest occupied orbital of the fluorescent dye is at an energy level equal to or higher than that of the polycyclic aromatic hydrocarbon, there is the problem that hole accumulation in the fluorescent dye occurs. In addition, this prior art basically relates to an emission layer, so that a combination thereof with other layers does not achieve the required effects in some cases.